<template>
  <div>
    <p>num = {{num}}</p>
    <p>double1 = {{double1}}</p>
    <input v-model.lazy="double2"/>
  </div>
</template>

<script>
export default {

    data(){
        return {
            num: 20
        }
    },
    computed: {        
        double2: {
            get(){
                return this.num * 2                
            },
            set(val){
                this.num = val / 5
            }
        },
        double1() {
            return this.num * 2
        }
        
    }

}
</script>

<style>

</style>